In older QGIS version (3.0 or before), OTB applications are available from QGIS.
Use them from the processing toolbox, which is accessible under Processing
-> ToolBox. Switch to “advanced interface” in the bottom of the
application widget and OTB applications will be there.

If QGIS cannot find OTB, the “applications folder” and “binaries folder”
can be set from the settings found under Processing
Settings “service provider”.

On some versions of QGIS, if an existing OTB installation is found, the
textfield settings will not be shown. To use a custom OTB instead of the
existing one, you will need to replace the otbcli, otbgui and library
files in QGIS installation directly.